PEDIGREES AND RELATIONSHIPS 25 



dinosaurs from a side branch of the stem connecting belodonts 

 with birds. According to this view the proximate reptilian 

 ancestors of birds still remain to be discovered. Possibly it 

 was in Africa that the annectant forms were developed. 



The last branch of the Reptilia is the one culminating in the 

 Squamata (snakes and lizards), which, like plesiosaurs and 

 chelonians, appears to be directly derived from the primitive 

 rhynchocephalian stock, with which it is connected by means 

 of the Acrosauria, a group provisionally admitted to ordinal 

 rank, and typified by the lizard-like A cvosaurus and Pleurosaurus 

 of the Upper Jurassic of the Continent. The Squamata have 

 either retained the ancestral acrodont dentition of the Rhyncho- 

 cephalia, or have modified this into the pleurodont type. They 

 have also inherited the ancestral clavicles ; but, on the other 

 hand, they have completely discarded the rhynchocephalian 

 plastron, or abdominal ribs and also the uncinate processes of the 

 true ribs. They have likewise lost the lower temporal arcade, 

 that is to say, the one connecting the quadrate by means of the 

 quadrato-jugal and jugal with the maxilla ; and probably cor- 

 related with the loss of this bar is the loosening of the attachments 

 of the quadrate itself, which is movably attached to the skull, 

 and thus makes a loose hinge for the lower jaw, which is of 

 special value to the pythons, boas, and other snakes which 

 gorge their prey in bulk. The complete scaling of the body 

 is, as its name implies, a very characteristic feature of the order, 

 although in some cases this has been lost. 



Next to fishes and amphibians (as represented by the Ste- 

 gocephalia), reptiles are the oldest vertebrates; in other words, 

 they are the oldest of the amniotic vertebrates. Bearing this 

 in mind, and also remembering the fact that (unlike fishes, 

 which are restricted to the water) during the Mesozoic epoch 

 they alone filled, in the main, the places now occupied in 

 nature conjointly by terrestrial and aquatic mammals, birds, 

 and reptiles, it is not surprising to find that, with the progress 

 of time, they have lost more heavily in ordinal groups than any 

 other vertebrate class. They have, in fact, been much more 

 than decimated, for out of the fifteen orders recognised in the 

 table on page 4 only four, as already said, now exist, and of 

 these one is represented only by a single genus and species, or, 

 at most, by two or three closely allied species of that genus. 



